An electrical contact brush assembly with anti-vibration head

ABSTRACT

An electrical contact brush assembly with a brush head and a brush body, the brush head being of resiliently deformable material of mechanical impedance of the same order as that of the brush body, and the brush head being in contact with a top face of the brush body to provide an interface therewith so that high frequency compressive sound waves can pass into the brush head from the brush body and be absorbed to reduce brush bounce. The brush head may be slotted.

United States Patent my Wiggs 1 Jan. 15, 1974 1 ELECTRICAL CONTACT BRUSH ASSEMBLY WITH ANTl-VlBRATlON HEAD [75] lnventor: Peter Kenneth Clifford Wiggs,

Tadworth, England [73] Assignee: Morganite Carbon Limited, London,

England [22] Filed: Apr. 26, 1972 [21] Appl. No.: 247,839

[52] US. Cl. 310/251 [51] Int. Cl. H02k 13/00 [58] Field of Search 310/251, 249, 248,

[56] u References Cited UNITED STATES PATENTS 3,525,006 Parr Ill Helwig 310/248 Helwig 310/248 2,923,842 2/1960 3,601,645 8/1971 Whiteheart ....3l0/251 3,392,295 7/1968 Sebok... ....310/251 3,484,686 12/ l 969 Wade 310/248 Primary Examiner-R. Skudy Attorney-Robert B. Larson et a1.

[57] ABSTRACT An electrical contact brush assembly with a brush head and a brush body, the brush head being of resiliently deformable material of mechanical impedance of the same order as that of the brush body, and the brush head being in contact with a top face of the brush body to provide an interface therewith so that high frequency compressive sound waves can pass into the brush head from the brush body and be absorbed to reduce brush bounce. The brush head maybe s1ot ted.

6 Claims, 4 Drawing Figures .SHEETEUFZ /,H I v ELECTRICAL CONTACT BRUSH ASSEMBLY WITH ANTI-VIBRATION HEAD This invention relates toelectrical contact brush assemblies, such as commutator or slip-ring brushes, and in particular relates to an improved brush assembly for reducing brush bounce.-

According to the invention, an electrical contact brush assembly comprises an electrically conductive brush body with a bottom face designed to contact a conductor and a top face in contact with a brush head formed from a resiliently deformable material having a mechanical impedance of at least two thirds and not more than two times that of the brush body so that the interface between the brush body and the head is substantially acoustically transparent to incident compressive elastic waves that may be generated at the brush body bottom face by intermittent contact with the conductor.

In a first embodiment of the invention, the brush head is a pad of heavy, resiliently deformable material, such as rubber, so that the mechanical impedance of the brush head is at least equal to that of the brush body.

in a second embodiment of the invention, the brush head consists of a pad of resiliently deformable material having a cross-sectional structure with an upwardly reducing cross-sectional area of material such that lat- I eral rigidity progressively decreases from the bottom face to the top face of the pad so as to promote absorption of elastic waves incident on the bottom face.

In a preferred form of the second embodiment the brush head consists of a pad of rubber having slots, or equivalent recesses, of different depths from the top face towards the bottom face.

Any compressive elastic waves generated at the brush body bottomface are likely to be predominantly plane longitudinal waves which can pass, with comparatively little or no reflection, through the brush body/- head interface into the brush head where they can be attenuated as they pass. up the head, reflected at the brush head top face and pass down the head back in to the brush body.

The ratio ofthe mechanical impedance of the brush head to that of the brush body should be unity for a minimum reflection of the compression waves at the brush body/head interface. In practice, a ratio in the range 2:3 to 2:1 is generally found to be acceptable. Above a ratio of 2:1 the compression waves incident on the interface will be increasingly reflected without change and below 2:3 the compression waves will be increasingly reflected as tension waves.

An air film at the interface would also increase reflection and, preferably, the brush head is physically bonded to the brush body by a thin adhesive layer to preclude such an air film.

The invention is illustrated, by way of example, on the accompanying drawings, in which:

FIG. 1 is an isometric view of an electrical contact brush assembly, according to a first embodiment of the invention,

FIG. 2 is a corresponding view ofa preferred form of a second embodiment of the invention,

FIG. 3 is a section on the line Ill [ll of FIG. 2, and a split brush assembly in accordance with the invention.

As shown in FIG. 1, a brush assembly 1 comprises a brush body'2, formed by a block ofelectrographite with a bottom face 3 curved to fit the surface of a commutator (not shown) and a plain top face 4, and a brush head 5, formed by a rectangular pad of high density, solid rubber with a plane bottom face 6, concave sides 7 and a plane top face 8.

The brush head bottom face 6 is bonded to the brush body top face 4 by a thin adhesive layer 9 of, for example, ARALDITE Trade Mark.

A cap 10, formed by a plate of rigid insulating material rests on the brush head top face 8 to bear the thrust of a brush holder spring (not shown).

In a second form, shown in FIGS. 2 and 3, the brush assembly 1 comprises a brush body 11 in the form of a block of electrographite material, with a curved bottom face 12 and a plain top face 13, and a brush head 14 in the form of a rectangular pad of solid synthetic rubber (NEOPRENE I'rade Mark) with a plane bottom face 15, concave sides 16 and a plane top face 17.

The brush head bottom face 15 is bonded to the brush body top face 13 by a thin adhesive layer 18.

A cap 19, formed from a rigid insulating strip, rests on the brush head top face l7 to bear the thrust of a brush holder spring (not shown).

Transverse slots 20 in the brush head extend to different depths respectively from the top face 17. The slots 20 progressively reduce the transverse rigidity of the brush head 14 from its bottom face 15 to its top face 17 by providing internal cavities into which the rubber can deflect sideways, to absorb energy. The adhesive bond may also lend some transverse rigidity to the brush head bottom face 15.

Investigation has shown that the majority of the clastic waves incident on the brush body/head interface arev plane longitudinal compression waves and that most of the energy of these waves is contained in the frequency band, lOOKHz to IMHZ. At these frequencies, the mechanical impedance of a high density, solid rubber can be equal to or greater than the mechanical impedance of an electrographite. Using these materials, a brush body/head interface can'be virtually, completely acoustisally .transrarcm and near y 109% 9f he. n5= q1t compression waves can pass into the brush head where a proportion of the waves will be absorbed by the brush head material as the waves pass up the head, the remainder being reflected at the brush head face to pass back down the head, with a further proportion being absorbed, before re-entering the brush body.

The particulars of three brush assemblies, made for purposes of comparison, are given below:

HEIGHT TRANSVERSE Brush Body 20.0 mm 25.4 mm X 19.05 mm Adhesive Layer 0.l mm Do. Brush Head l0.0 mm Do. Cap L6 mm Do. Total Height 3L7 mm All the brush bodies were formed of an electrographite having a density of l .6 Mg m' and a mechanical impedance of 3,000 Mg m" 5' Brush head A was formed from a conventional, solid rubber having a density of nearly l Mg m and a mechanical impedance of 1,500 Mg m 8", brush head B was formed from a solid rubber having a density of 4.2 Mg m and a mechanical impedance of between 3,000 and 4,000 Mg m" S", and a brush head C was formed of the same material and size as brush head A but slots were cut, 1 mm wide and of depths from 4 mm to 9 mm in the top of the head.

On test, a reduction in brush bounce time of approximately 16 was observed between the brush assemblies with heads B and C and the assembly with head A. Brush bounce time being defined as the time interval between the first contact and the last break in contact after a brush is allowed to slide off a 0.25 mm high step.

We have found that the best results are obtained by using a resiliently deformable material for the brush head, having the same mechanical impedance as the brush body material. If it is not possible precisely to match the mechanical impedance of the brush head material with that of the brush body material, we believe it is usually preferable to have a mismatch where the mechanical impedance of the brush head material is greater than that of the brush body material.

In an example, a brush head having the same dimensions and formed from the same material as brush head B, was cut with the same slots as brush head C. For some applications we have found that such a brush assembly having a slotted heavy rubber head shows a further reduction in brush bounce over a brush assembly having a plain heavy rubber head or a slotted head of conventional rubber. The impedance match between the brush head and the body materials being the same for both assemblies.

Although slots of different depths have been described, a similar effect could be obtained by cylindrical holes of different depths or conical holes of uniform depth.

FIG. 4 shows a split brush assembly 21 comprising a pair of brush bodies 22 and 23 to each of which is bonded a brush head 24 and 25 respectively. A single cap 26 rests on both brush heads and transverse slots 27 of different depths are cut in both brush heads.

A heavy top 28, for example of brass, rests on the cap 26 and may be permanently attached to a brush holder spring (not shown). The top 28 increases the inertia of the brush assembly 21 and, acting with the resilient brush heads 24 and 25, hastens response in returning the brush bodies into contact with a conductor for low frequency small and short-time displacements of the brush bodies (for example, a 50 Hz vibration caused by an eccentrically running commutator). The heavy top is equally effective for the brush assemblies 1 (FIGS. 1, 2 and 3).

l claim 1. An electrical contact brush assembly comprising an electrically conductive brush body, a bottom face on said body designed to contact a conductor and an antivibration bush head formed from a pad of resiliently deformable material in contact with a top face of said body, wherein the mechanical impedance of said brush head is between two thirds and twice the mechanical impedance of said brush body to provide an interface between said brush body and said brush head with low resistance to incident compressive elastic waves, and said pad has slots therein of different depths from a top face of said brush head towards a bottom face of said brush head, to promote absorption of said compressive elastic waves by said brush head.

2. An assembly as claimed in claim I, wherein said brush head is physically bonded to said brush body by a thin adhesive layer.

3. An assembly as claimed in claim 2, wherein a cap rests on said brush head top face and a heavy mass rests on top of said cap.

4. An electrical contact brush assembly comprising an electrically conductive brush body, a bottom face on said body designed to contact a conductor and an antivibration brush head formed from a pad of heavy, resiliently deformable material in contact with a top face of said body such that the mechanical impedance of said brush head is at least equal to that of said brush body to provide an interface between said brush body and said brush head with minimal resistance to incident compressive elastic waves, wherein said pad has slots therein of different depths from a top face of said brush head towards a bottom face of said brush head to promote absorption of said compressive elastic waves by said brush head.

5. An assembly as claimed in claim 4, wherein said brush head is physically bonded to said brush body by a thin adhesive layer.

6. An assembly as claimed in claim- 5, wherein said cap rests on said brush head top face and a heavy mass rests on top of said cap.

44-1---DETA(" II HERE BEFOREJMMLING Tm: TYIED CERTIFICATE TO THE PATENT nrrlc'e "j UNIl IJD' STA'IES PATE T F ICE CERTIFICATE OF CORRECTION Patent 3,786 ,292" Dated January 15, 1974 Invent-Cd's)" Peter K; c. wi gs- I It i s -cer tified "etror appears in the aboveidentified .oatent and that said Letters Patent are hereby corrected as shown below:

Please atitlthe following totthe heading o f t he patent: v [30]"Foreign:applioation priority data a I May 7, 197i Great BritainHQ... .....-l3608/7l Ju nel6 1971 Great B ritainn'n.....28l82/7l Signed and sealed this 2nd day of July 1974,

(SEAL) Attest:

C.MARSHALL DANN Commissioner of Patents EDWARDIM. .--FLETIC-HER,JR".V Attesting Officer DETAQH HERE nEruRE. MAILING THE TYPED CERTIFICATE TO THE PATENT ornc'r;

4 UNITED S'IAIES'fPA'lENl OFFICE CERTIFICATE OF CORRECTION l 3,786 ,292 l a Dated" January (15, 1 97 4 Inveht o r Pter 'K. C wigijs,

It is c er t-ifie d that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Please add the following to the heading of th patent:

Foreigr app'lication priority data May 1971 Great BritainHQ........-l3608/7l Jimelfi, "1971 Great Britain;..-...,...28l82/7l Y Sign ed and saled this 2nd day of July 1974,

(SEAL)- Attest:

EDWARD FLE-TCHERJR, QMARSHALL DANN Att es fing Officer i Commissionrbf P atnts 

1. An electrical contact brush assembly comprising an electrically conductive brush body, a bottom face on said body designed to contact a conductor and an anti-vibration bush head formed from a pad of resiliently deformable material in contact with a top face of said body, wherein the mechanical impedance of said brush head is between two thirds and twice the mechanical impedance of said brush body to provide an interface between said brush body and said brush head with low resistance to incident compressive elastic waves, and said pad has slots therein of different depths from a top face of said brush head towards a bottom face of said brush head, to promote absorption of said compressive elastic waves by said brush head.
 2. An assembly as claimed in claim 1, wherein said brush head is physically bonded to said brush body by a thin adhesive layer.
 3. An assembly as claimed in claim 2, wherein a cap rests on said brush head top face and a heavy mass rests on top of said cap.
 4. An electrical contact brush assembly comprising an electrically conductive brush body, a bottom face on said body designed to contact a conductor and an anti-vibration brush head formed from a pad of heavy, resiliently deformable material in contact with a top face of said body such that the mechanical impedance of said brush head is at least equal to that of said brush body to provide an interface between said brush body and said brush head with minimal resistance to incident compressive elastic waves, wherein said pad has slots therein of different depths from a top face of said brush head towards a bottom face of said brush head to promote absorption of said compressive elastic waves by said brush head.
 5. An assembly as claimed in claim 4, wherein said brush head is phySically bonded to said brush body by a thin adhesive layer.
 6. An assembly as claimed in claim 5, wherein said cap rests on said brush head top face and a heavy mass rests on top of said cap. 